As a consequence of recent efforts to thwart the recent upsurge in international terrorism, there is an increased interest in the detection of explosive materials. These materials include nitro-aromatic compounds including 2,4,6-trinitrotoluene (TNT), dinitrotoluene (DNT) and similar derivatives.
Many detection methods have been used to detect explosive materials. These methods include gas and HPLC chromatography, x-ray scattering, neutron analysis, nuclear quadrupole resonance, and mass spectrometry (U.S. Pat. No. 6,571,649). These methods generally require expensive and sophisticated equipment, (e.g. high vacuum), equipment that is not portable (e.g. cylinders of compressed gases), and/or have a complicated sample preparation. These techniques, are therefore, not appropriate for low cost portable field-testing for trace explosive materials. A recent review of some of these methods for explosives detection is “Explosives detection systems (EDS) for aviation security” (Singh, S., Signal Processing vol. 83, 2003, p. 31-55).
Another known method for the detection of trace amounts of explosive materials utilizes immunochemical sensors. For example, U.S. Pat. No. 6,573,107 is directed towards the immunochemical detection of explosive substances in the gas phase using surface plasmon resonance spectroscopy. Immunochemical detection methods potentially offer high selectivity and high sensitivity.
Electrochemical detection refers to the use of electrodes, immersed in an electrolyte, and connected to an instrument that varies the voltage applied to the electrodes. The instrument measures the current flow between the electrodes. Typically, the electrode potential is varied; and an electric current flows between the electrodes that is characteristic of the presence of electrochemical active substances in the electrolyte. The magnitude of the current is proportional to the concentration of the electrochemical active substances. It is well known that TNT and other nitro-aromatic compounds are reduced electrochemically at the cathode and may be detected by electrochemical detection. Wang et. al. (Analytica Chimica Acta, vol. 485 (2003) p. 139-144) reported the monitoring of TNT in natural waters using an electrochemical technique. They reported a measurement sensitivity of 0.003 μA/ppb of TNT in natural seawater. This sensitivity level was achieved by Wang et. al. by subtracting the background signal, in natural seawater not contaminated by TNT, caused by the reduction of dissolved oxygen. The applicant reported (Reviews Analytical Chemistry vol. 18 no. 5, 1999, p. 293) the use of carbon/Hg film electrode materials in an aqueous solvent. This electrode material was successful to minimize the background by separating the atmospheric O2 background current from the TNT current, however the sensitivity reported was only ˜0.7 μA/μM (˜0.003 μA/ppb) and was comparable to the sensitivity reported by Wang. Despite these positive developments in the prior art, the sensitivity is still insufficient, and the kinetics of the TNT reduction reaction are too slow to achieve a practical portable field test for trace explosive materials. A practical electrochemical sensor for trace explosive materials should have high sensitivity, a short measurement time and in addition a way of cleaning the electrodes rapidly to perform further testing.
There is thus a widely recognized need for an electrochemical method and sensor for the detection of traces of explosives, and it would be highly advantageous to have an electrochemical method and sensor for the detection of traces of explosives, with high sensitivity, and fast reaction kinetics.